Producing N-vinylpyrrolidone polymers by free-radical polymerization is known. The polymerization mechanism under various conditions is described for example in Polymer Journal, 17, 143-152 (1985). The patent literature describes not only polymerizations in organic solvents as in alcoholic solution in U.S. Pat. No. 4,053,696 for example but also in aqueous solution as described in U.S. Pat. No. 2,335,454 for example.
Polymerization in organic solvents as described in U.S. Pat. No. 4,053,696 generally leads to products comprising only a relatively low proportion of impurities such as formic acid. But this process has the immense disadvantage that the polymerization first has to be carried out in a solvent such as isopropanol. Only after the polymerization has ended, a costly and inconvenient distillation process has to be used to distill off the solvent e.g. the isopropanol and replace it with water. This generates comparatively large amounts of solvent, which either have to be disposed of or purified by distillation. This leads to long occupancy of the polymerization reactor and an unfavorable space-time yield.
Polymerization of N-vinylpyrrolidone in aqueous solution, by contrast, is usually carried out in the presence of hydrogen peroxide as initiator, as described in U.S. Pat. No. 2,335,454 for example. The molecular weight of the polyvinylpyrrolidone here depends on the hydrogen peroxide concentration in that low molecular weights are the result of high hydrogen peroxide concentrations, and vice versa. But high hydrogen peroxide quantities promote the formation of formic acid in the aqueous system; polymers thus obtained have a distinct yellow coloration even in the as-polymerized state.
DE 11 2005 002 719 describes a process for producing aqueous polyvinylpyrrolidone polymer solutions. A process for handling vinylpyrrolidone polymer solutions is also disclosed. This process provides polymers of low HAZEN color number (the Hazen color number corresponds to the “cobalt-platinum color number” and is known per se to a person skilled in the art, for example from the industrial standard DIN ISO 6271-1). The polymer solutions obtained by this process have the immense disadvantage of a high formic acid content and of gradually discoloring in the course of storage. The formic acid is by-produced as an undesired secondary component during the polymerization. The polymer obtained is unsuitable for frequent purposes, especially when PVP powders are produced from the solutions by spray drying. There are formic acid limits to be heeded for use in pharmaceutical and cosmetic products in particular. For instance, a formic acid limit of not more than 0.5 weight percent is prescribed in the “Povidones” monograph in the European Pharmacopeia, and even in cosmetic formulations, the formic acid fraction present as preservative must not exceed a limit of 0.5 weight percent. As mentioned, the initially colorless solutions discolor on prolonged storage.
But this property of discolorations is precisely what is undesirable for cosmetic formulations, especially when transparent, colorless hair gels are to be produced therefrom.
Bühler reports color changes in aqueous PVP solutions especially after storage or heating, for example in the course of sterilization. The resulting yellow to yellowish brown coloration results from oxidation due to atmospheric oxygen. This, according to Bühler, can be avoided by adding suitable antioxidants. Bühler names cysteine and sodium sulfite as such antioxidants (Volker Bühler, “Polyvinylpyrrolidone—Excipients for Pharmaceuticals”, Springer, 2005, pages 34 and 35 regarding stability in solid and liquid dosage forms).
The peroxides from the polymerization and formed directly thereafter have the disadvantageous effect of being at least partly consumed even on addition to the polymer thus reducing the protection and the length of storage. To compensate this effect, therefore, comparatively large amounts of antioxidant have to be used.
The oxidation sensitivity of polymers such as PVP, the macroscopically visible and measurable effects of oxidation and also proposed measures to control and inhibit oxidation has been described in many publications (see for example Bühler in the above-cited publication; Kline in Modern Plastics, 1945, November, from page 157; Reppe in the monograph regarding PVP, Chemie Publishers, Weinheim, 1954, page 24; EP-B 873 130; U.S. Pat. No. 6,331,333; U.S. Pat. No. 6,498,231; Staszewska in “Die Angewandte Makromolekulare Chemie”, 1983, 118, pages 1 to 17).
U.S. Pat. No. 2,821,519 describes a process for stabilizing PVP via addition of hydrazine and derivatives thereof. Hydrazines are toxicologically concerning and undesired in polyvinylpyrrolidones, N-vinylpyrrolidone copolymers and polymers of N-vinylpyrrolidone derivatives in particular.
EP-B 1 083 884 describes a process for stabilizing polyvinylpyrrolidones against peroxide formation. Aqueous solutions of the polymers are admixed with very small amounts of heavy metal salts or with peroxide-cleaving enzymes. These remain in the product. Suitable heavy metals are manganese, zinc, cobalt and especially copper.
However, the use of the heavy metals proposed is disadvantageous by reason of possible accumulation in the body. The use of enzymes is disadvantageous for cost and stability reasons.
GB 836,831 discloses a process for stabilizing polyvinylpyrrolidones against discoloration wherein solutions of the polymers are treated with sulfur dioxide, sulfurous acid or alkali metal sulfites. It is disclosed to add the sulfur compound by mixing it at room temperature into polymer solutions obtained by dissolving dry polymeric powder in water. The sulfur compound is used as a reducing agent which is said to offer protection from yellowing at high temperatures to which the polymer solution is exposed in the course of undergoing sterilization or drying.
DE 10 2005 005 974 discloses that in the process known from GB 836,831 the peroxide buildup by storage occurs to an even greater degree than in untreated polymers. DE 10 2005 005 974 discloses a process wherein the polyvinylpyrrolidones are first treated with sulfur dioxide, sulfurous acid or alkali metal salts thereof and then with a free-radical scavenger. The sulfur-containing reagents are added after the postpolymerization and after any acidic hydrolysis and immediately before the optional drying, incorporated by stirring and serve as reducing agent. Antioxidants have to be used in addition to achieve any stabilization.
Residual monomer is reduced in the prior art by repeated addition of minor initiator portions to the polymer solution after the polymerization and postpolymerization (DE 11 2005 002 719). This described aftertreatment of DE 11 2005 002 719 involving renewed addition of initiator, however, leads to elevated and unwanted formate contents.
Residual monomer reduction by addition of organic and inorganic acids is known from WO 93/16114 A1. It discloses reduction to pH below 5 for acidic hydrolysis of vinyllactams to free lactams, such as vinylpyrrolidone to 2-pyrrolidone.